Treatment of single functional ventricle is indisputably a significant healthcare challenge. It is the leading cause of death from any birth defect in the first year of life. Those who are fortunate to survive face lifelong disability for which there is no direct therapy, constituting an emerging public health concern. Currently, repair is performed in a complex series of 3 staged operations which are notorious for instability and mortality. This is attributable to the use of a systemic arterial shunt to provide a substantive source of pulmonary blood flow in the neonate. The endpoint of repair is a univentricular Fontan circulation, in which the vena cavae are connected to the pulmonary artery. Because there is no right-sided ventricular power source, venous return is profoundly altered and filling of the single ventricle is suboptimal. We have theorized that a means to modestly augment existing Fontan cavopulmonary flow (6-8 mmHg) would address these problems by reproducing more stable two-ventricle physiology and permitting stabilization and/or compression of surgical stages. Gradual reduction in support would permit adaptation to the higher pressure needed (10-15 mmHg) for a systemic venous source to independently perfuse the lungs. The functional parameters for a blood pump to provide low- pressure support in the complex 4-way flow anatomy of a cavopulmonary connection are markedly dissimilar to any other circulatory assist application: No such pump currently exists. We hypothesize that an actuator disk pump, based on the von Karman viscous pump, is optimal to provide cavopulmonary assist. With only one impeller, a catheter-based expandable rotary disk provides the preferred low-pressure, high-volume flow in 4 opposing directions without risk of venous pathway obstruction. To develop this breakthrough innovation, our specific aims are to: 1) characterize the upstream, local, and downstream flow patterns induced by rotation of a central stabilizing body (actuator disk) within a 3-way T and 4-way t cavopulmonary connection, 2) define the optimal geometric and surface characteristics of a bi-conical expandable rotary impeller to augment cavopulmonary flow using advanced numerical modeling and flow visualization, 3) optimize the hemodynamic, biocompatibility, and thrombogenicity performance of a rotary disk pump through in vitro feedback from mock flow loop, flow visualization, and hemolysis studies, 4) demonstrate percutaneous viscous pump support in an animal model of univentricular Fontan circulation. We will accomplish these aims by intersecting expertise in: computational fluid dynamic modeling; surface streamlining; flow visualization; in vitro modeling; physiologic control; thrombogenicity; elastomer chemistry; nitinol metallurgy; microcoil fabrication; catheter disposables; rotary blood pump design; prototyping; and clinically rooted in vivo studies. At completion, an easily implemented percutaneous technology which dramatically improves Fontan circulatory status will be delivered as a predicate device to clinical use. In patients with univentricular Fontan circulations - young and old - this safe, simple, and reliable method to augment cavopulmonary flow will address their unresolved health needs.